1. Field of the Invention
This invention relates generally to determining the quality and/or the heat value of a condensable vapor; and more particularly to methods and apparatus for continuously monitoring the quality and/or the heat value of a flowing vapor such as steam by representative sampling, calorimetric measurement, and mass flow measurement.
2. Description of Prior Art
The quality of condensable vapor, such as steam, is ordinally determined by a calorimeter. The most common types of the steam calorimeter, arranged according to their known accuracy, are: throttling, separating, combined throttling and separating, electric, and barrel calorimeters. Except the separating calorimeter, the common process of these calorimetric methods is that the measurements are made at a lower energy level than that of the original sampling conditions and that it is assumed that the kinetic energy of the high-velocity steam is all returned as heat in the fluid when it is brought to rest in the low-pressure downstream calorimeter chamber for the determination of quality. In case of the throttling calorimeter, the readings can be used if there is superheat at this point. If there is no superheat present in the calorimeter chamber at atmospheric pressure, the chamber must be connected to a vacuum source to extend the range of expansion; or a device, such as a separating calorimeter, must be used in which the liquid is mechanically separated from the vapor and measured. Since it it not always clear whether or not superheat is present in the calorimeter chamber of a throttling calorimeter, and the accuracy of the separating calorimeter is unsatisfactory, the limits of moisture within which the throttling calorimeter will work at sea level are known to be from 2.88% at 50 psi to 7.3% at 400 psi. Furthermore, at least 10.degree. F. superheat must be obtained by the expansion for results within .+-.50% accuracy. Therefore, the throttling calorimeter is considered inadequate for determining steam quality at pressures above 400 psi.
The above common types of steam calorimeter require bleeding off a side stream of vapor sample, which is led to the atmosphere after passing through the calorimeter chamber. As a result, the measurements taken may be erroneous if a thermal equilibrium state has not been reached within the calorimeter before taking readings. The hot vapor sample bled off can cause a nuisance to the operator nearby and represents a loss of the vapor also. The barrel calorimeter involves a considerable expenditure of time and labor and it is not suitable for rapid determination of the vapor quality of transient flow systems. The conventional electric calorimeter for the intermittent determination of steam quality or the continuous flow type electric calorimeter has a serious drawback in common--both assume that the latent heat of vaporization and/or the specific heat of vapor is independent of temperature and pressure in determination of vapor quality. This inventor has found that both latent heat of vaporization and specific heats of all vapors are strongly dependent on temperature changes although they are less sensitive to pressure changes, as will be seen from the correlations developed by the inventor and disclosed hereinafter.
It is important to point out that in order to obtain accurate results calorimeters should be made to obtain a truly representative sample of the flowing vapor. The design of sampling device used and its location are important factors in obtaining a representative sample. The Power Test Code of the American Society of Mechanical Engineers recommends a sampling nozzle made of 1/4 or 3/8-inch pipe, closed at the inner end and having the portion of the nozzle inside the steam main drilled with 1/8-inch holes spaced on 1/2 inch centers for steam mains up to 6 inches in diameter. This sampling nozzle should be located in the main in such a position that the holes will directly face the steam flow. The above design of sampling nozzle has become a standard practice in the industry although it is doubtful whether a representative sample is always obtained using a sample nozzle of the above design, as the composition of the total sample taken is affected by several factors including the radial distribution of entrained liquid, the vapor pressure and velocity, the pressure drop of vapor sample entering each sampling hole, etc. All these factors have not been fully taken into consideration previously in the design and location of sampling nozzle.
In addition to the calorimetric methods described above, many two phase flow measurement techniques, which may be related to the determination of vapor quality, have been investigated and evaluated by the present inventor. These include the measurement techniques of pressure, pressure differential, fluid temperature, density and void fraction, flow regime and void distribution, velocity slip, etc. The results of the above investigation and evaluation indicate that while some categories of these techniques require the development of implementation designs and operating techniques in order to avoid the adverse effects of the normally hostile environment of the transient two-phase, high-pressure, high-temperature conditions associated with power plant safety tests, other categories need considerable developments and improvements in the actual design and the fundamental understanding of the operating characteristics. None of the above two-phase flow measurement techniques was found to be capable of accurately monitoring the quality and enthalpy of a condensable vapor flow.